1. Field of the Invention
The present invention relates to method of searching reverse traffic channels in a code division multiple access (CDMA) mobile radio communication system and more particularly to a method of searching reverse traffic channels for allowing for acquisition of reverse traffic channels without increase or decrease of size of a search window used for acquisition of a traffic channel when pseudonoise (PN) random delays are employed for reverse access channels and increase or decrease, by changing a location of the search window through a control method of shifting the center of the search window.
2. Description of Related Art
Generally, a mobile radio communication system is a communication system for mobile units such as human beings, automobiles, ships, trains, and aircraft and includes a mobile telephone (a portable telephone and automobile telephone), harbor telephone, aircraft telephone, mobile pay phone (which is installed at a train, sightseeing boat, express bus, and the like), radio paging, cordless telephone, satellite mobile radio communication, amateur radio communication, and fishery radio communication.
Such mobile radio communication has been implemented through an advanced mobile phone service system using an analog, a CDMA system using a digital, a time division multiple access (TDMA) system, and a frequency division multiple access (FDMA) system.
The CDMA system comprises: a mobile station 1 selectively or simultaneously supporting the services of the AMPS and CDMA systems, for allowing subscribers to communicate through a mobile communication network; a base transceiver subsystem 2 for performing communication with the mobile station 1 in a radio area using a protocol defined in the IS-95; a base station controller 3 for controlling a radio link and a cable link and performing an hand off function for maintaining continuity of a call even while the mobile station 1 is moving; a mobile switching center 4 for forming a speech path for a subscriber and performing access to another communication network; and a home location register 5 containing subscribe information, for transmitting and receiving signals to and from the mobile switching center 4. The CDMA system realizes the call of the mobile station 1 through the base transceiver subsystem 2, the base station controller 3, and the mobile switching center 4.
The home location register 5 is a system for storing the subscriber information.
In such general CDMA mobile radio communication system, all mobile stations and base transceiver subsystems adjust their time sync on the basis of the CDMA system reference time.
Specifically, the mobile station (terminal equipment) acquires time information of the long code timing and system timing of the CDMA system from a sync channel message received from the base transceiver subsystem and respectively synchronizes the long code timing and the system timing with its long code timing and system timing. Through such synchronization, the mobile station reconstitutes signals transmitted from the base transceiver subsystem, and base transceiver subsystem reconstitutes signals transmitted from the mobile station.
The procedure that the mobile station acquires the time information from the message transmitted from the base transceiver subsystem and then establishes a reference time so as to be synchronous to the base transceiver subsystem will now be described in detail.
There exist a pilot channel, a sync channel, a paging channel, and a forward traffic channel in a forward link (from the base transceiver subsystem to the mobile station), and there exist an access channel and a reverse traffic channel in a reverse link (from the mobile station to the base transceiver subsystem). The mobile station acquires the pilot channel and is synchronous to the acquired pilot channel so as to acquire the sync channel (sync channel message). The mobile station acquires the time information from system time data, SYS_TIMEs, contained in the sync channel message. The system time data, SYS_TIMEs, is the system time at a point obtained by subtracting a pilot PN offset from a point beyond 320 ms which is the end of a last 80 ms super frame in the sync channel message received at the mobile station.
The reference time of the mobile station is established to a point obtained by adding the signal propagation delay from the base transceiver subsystem to the mobile station and the signal process delay at the mobile station to the reference time of the base transceiver subsystem. The mobile station transmits a signal at such reference time established as described above. The CDMA system generates and extracts the signals with a factor of time, so the mobile station should establish the reference time and transmits the signal in synchronization with the established reference time. This reverse link signal is delayed by the propagation delay before being received at the base transceiver subsystem. In other words, the base transceiver subsystem transmits the signal in synchronization with the system time, and the reception of the reverse link signal at the base transceiver subsystem is delayed (this delay is called hereinafter "bothway propagation delay").
The base transceiver subsystem searches a reverse access channel of the mobile station while time-delaying on the basis of the system time. The access channel is used before making a call (traffic channel) between the mobile station and the base transceiver subsystem and contains many messages such as a registration message, an order message, a data burst message, an origination message through which the mobile station requests an originating call, a page response message which is a response message to an incoming call, and an authentication challenge response message.
When the base transceiver subsystem acquires the access channel sent by the mobile station while searching the reverse access channel, a time offset at this point corresponds to the bothway propagation delay which is the time delay from the base transceiver subsystem via the mobile station back to the same base transceiver subsystem. The mobile station receives a channel assignment message which the base transceiver subsystem sends out after acquiring the access channel. Then the mobile station sends a traffic channel preamble to the base transceiver subsystem to maintain a link with the base transceiver subsystem. The traffic channel preamble is sent to the base transceiver subsystem to aid the base transceiver subsystem in performing initial acquisition of a reverse traffic channel and consists of frames of 192 zeros at the 9600 bps rate. The base transceiver subsystem establishes a search window centering around the offset (the bothway propagation delay on the basis of the system time) obtained when the access channel is acquired and then searches the reverse traffic channel preamble.
When multiple mobile stations are located in the same place or the places which are distant the same propagation delay from the base transceiver subsystem within the coverage area of the base transceiver subsystem, the mobile stations send messages to the base transceiver subsystem on the access channels at the same time. The messages from the different mobile stations are received at the base transceiver subsystem at the same time, and therefore, collision of the access channels occurs. Consequently, the base transceiver subsystem cannot demodulate the messages transmitted from the mobile stations. In other words, since each mobile station uses its unique long code, the reverse traffic channels from multiple mobile stations are distinguishable, but the access channels use the same long code, so they are indistinguishable. In this regard, if the messages are simultaneously received at a receiver of the base transceiver subsystem, collision between the access channels occurs.
To solve this problem in the CDMA mobile radio communication system, that is, to prevent the collision between the access channels from multiple mobile stations at the base transceiver subsystem when transmitting the access channels on the reverse links (from the mobile stations to the base transceiver subsystem), the base transceiver subsystem randomly establishes time delays for the reference time with respect to each mobile station (hereinafter, this will be called a PN random delay) so that the mobile stations can transmit the access channel data at different times given in accordance with each PN random delay, preventing collision between the access channels at the receiver of the base transceiver subsystem. Such PN random delay is produced through a hash function using an electronic serial number (ESN) of the mobile station (IS-95 Spec, 6.6.7).
FIG. 2 is a diagram for describing a conventional method of searching a reverse traffic channel in the CDMA mobile radio communication system.
With reference to FIG. 2, the conventional reverse traffic channel search method will now be described. A mobile station transmits an access channel to the base transceiver subsystem using a PN random delay, so transmission of the access channel is delayed to a time point 30 which is established by the PN random delay 20 produced in accordance with the ESN of the mobile station on the basis of an access channel slot boundary 10 arranged at the system time.
The access channel slot boundary 10 indicates an access channel slot and frame boundary when the PN random delay is not used.
When considering on the basis of the system reference time 40 of the base transceiver subsystem, the access channel is received at the base transceiver subsystem at a time point 70 resulting from a delay by the sum of the bothway propagation delay (sum of a propagation delay 50 from the base transceiver subsystem to the mobile station and a propagation delay 60 from the mobile station to the base transceiver subsystem) and the PN random delay 20. An offset 80 after the base transceiver subsystem acquired the access channel corresponds to the sum of the bothway propagation delay and the PN random delay.
Once the mobile station receives a channel assignment message transmitted from the base transceiver subsystem which has acquired the access channel, it transmits a traffic channel preamble to the base transceiver subsystem at a mobile station system reference time 90 to aid the base transceiver subsystem to easily acquire a reverse traffic channel at the time of initial access. The PN random delay is a delay time used for preventing an access collision, so it is used only for the access channel but not used for the transmission of the traffic channel preamble.
The reverse traffic channel preamble has the propagation delay 60, thus being received at the base transceiver subsystem at a time point 110 resulting from a delay by the bothway propagation delay on the basis of a base transceiver subsystem's reference time 100. The base transceiver subsystem sets a center of a search window 130 to a time point 120 resulting from a delay by an offset 80 obtained at the time of acquisition of the access channel on the basis of a system reference time 100 and searches a reverse traffic channel to acquire a reverse traffic channel preamble.
In other words, the center 120 of the traffic channel search window of the base transceiver subsystem is located at the point obtained by adding the PN random delay 20 to the point 110 at which the reverse traffic channel preamble is received at the base transceiver subsystem (the point 110 resulting from the bothway propagation delay on the basis of the system reference time 100).
When a half size 140 of the search window does not exceed the PN random delay 20, the reverse traffic channel preamble cannot be acquired, resulting in fail in making a call. Therefore, the half size 140 of the search window must be greater than the PN random delay 20 to success in making a call.
To satisfy this condition, conventional technology establishes the search window by manually adjusting its size to make the half of the search window size larger than the PN random delay in order to search reverse traffic channels.
According to such conventional technology, as the PN random delay time gets greater, the size of the search window should be greater relative to the delay time. Under these conditions, it takes much time to search traffic channels, thereby deteriorating system performance.
Furthermore, since the size of the search window should be manually increased or decreased as the PN random delay increases or decreases, the conventional technology has drawbacks of wasting time and human resources.